When producing paper pulp by the kraft method, a spent liquor is obtained, which is generally called black liquor. The black liquor contains organic material and the residual chemicals from digesting the fibrous raw material. This black liquor is generally evaporated and conveyed to a separate processing stage for recovery of the energy content of the organic material and also for recovery of the cooking chemicals. The cooking chemicals are usually recovered in a liquor, called green liquor. The well known Tomlinson process has been the commercially predominant method for this recovery of energy and chemicals. However, a disadvantage of the Tomlinson process is that it requires very large combustion furnaces which are complicated from the technical and operational points of view.
Swedish Patent SE-C448 173 describes a more modern process which, in addition to considerably simplifying the necessary process equipment, achieves improved recovery of both energy and chemicals. This process is based on a pyrolysis reaction in which the black liquor is gasified in a Chemrec.RTM. reactor. The process produces an energy-rich gas principally comprising carbon monoxide, carbon dioxide, methane, hydrogen and hydrogen sulphide. Inorganic chemicals are also produced in the form of small drops of smelt, principally comprising sodium carbonate, sodium hydroxide and sodium sulphide. The resulting mixture of gas and drops of smelt is quickly cooled in a first stage by direct contact with a cooling liquid. Green liquor is formed when the smelt chemicals and the hydrogen sulphide are dissolved in the cooling liquid. The gas is thereafter washed in a second stage in a scrubber type gas washer. The gas is then used as fuel for generating steam and/or electrical power. The physical thermal value of the gas can also be used when the gas is cooled from the gasification temperature to the saturation temperature for producing steam at the selected pressure. The green liquor which is formed is conveyed to a causticizing stage for production of white liquor.
SE-B468 600 discloses a process for producing white liquor of high sulphidity (a high proportion of sodium hydrogen sulphide in relation to the proportion of sodium hydroxide) directly from the gasification reactor and without any need for causticizing. In this process, hydrogen sulphide is recovered from the gaseous phase exhausted in the Chemrec.RTM. reactor and is returned to the reactor in order to be present during the thermal decomposition of the spent liquor. In this way, such a high partial pressure of hydrogen sulphide is established in the gasification stage that the following equilibrium reaction (a) EQU Na.sub.2 CO.sub.3 +H.sub.2 S.fwdarw.Na.sub.2 S+CO.sub.2 +H.sub.2 O(a)
is displaced so far to the right that the formation of Na.sub.2 CO.sub.3 is substantially suppressed. The Na.sub.2 S formed is dissociated to form NaOH and NaHS.
The recovery of hydrogen sulphide from the gaseous phase takes place by passing the gas through a gas washer containing an external absorption chemical, for example N-methyl-pyrrolidone or methyldiethylamine, for selective and regernative absorption of the H.sub.2 S present in the gas. The requirement for such an external chemical represents a significant disadvantage of this process and additionally makes it necessary to provide an undesirable regeneration stage for driving off the hydrogen sulphide from the absorption chemical.
Swedish Patent SE-C465 039 describes a method which has the object of producing a cooking liquor having a high sulphidity. Material containing sulphur and/or material containing sulphur and sodium, which generally occur in the pulp mill, are conveyed to a reactor together with the black liquor.
A general disadvantge of previously known recovery techniques is that the recovery of chemicals has been governed by the available process technology. For example, the sulphidity of the white liquor has been adapted to the requirements of the recovery boiler and not to the requirements of the cooking department. Nor has there been any commercially feasible technique for producing liquors having differing properties for different uses in the pulp mill.